Specific and high-affinity binding agents are indispensable tools for biological and medical research and also have utility for medical diagnosis, prophylaxis and treatment. At present, monoclonal antibodies are the predominant class of binding molecules that can be rapidly isolated with high affinity and specificity to virtually any target. However, immunoglobulins have limitations that are based mostly on their general biophysical properties and their rather complicated molecular structure. Therefore, already in the 1990's several research groups have explored small globular proteins as substitutes for antibodies. The idea behind this concept is the transfer of a universal binding site from an antibody structure to alternative protein frameworks, the so-called scaffolds. So far more than 40 scaffolds have been described, among them two SH3 domains, the SH3 domains of the Abl and the Src kinase (see Binz et al., Nature Biotechnology, Vol. 23, No. 10, 1257-1268, 2005).
SH3 domains are found in many different proteins involved in intracellular signalling and cytoskeletal organization (Cohen et al., “Modular binding domains in signal transduction proteins.” Cell 80(2): 237-48, 1995). Despite the variability in their primary structures these SH3 domains share a very similar overall structure and mode of binding to proteins sharing the minimal consensus sequence PxxP that is a critical determinant for natural SH3 binding. An important function of SH3 domains is to participate in highly selective protein-protein interactions.
Erpel et al. (“Mutational analysis of the Src SH3 domain: the same residues of the ligand binding surface are important for intra- and intermolecular interactions.” Embo J. 14(5): 963-75, 1995) investigated the influence of mutations in the RT and n-Src loops of Src SH3 domains and demonstrated that mutations in both loops which are adjacent to the hydrophobic surface could influence the ability of this domain to participate in inter- and intramolecular associations.
Hiipakka et al. (“SH3 domains with high affinity and engineered ligand specificities targeted to HIV-1 Nef.” J. Mol. Biol. 293(5): 1097-106, 1999) investigated the ability of the RT-loop of the Hck SH3 domain to act as a versatile specificity and affinity determinant. The authors constructed a phage library of Hck domains, where 6 amino acids of the RT-Loop were randomized (termed RRT-SH3). Using this strategy they identified individual RRT-SH3 domains that can bind to HIV-1 Nef up to 40 times better than Hck-Sh3. The authors indicate the importance of the RT loop in SH3 ligand selection as a general strategy for creating SH3 domains with desired binding properties.
Lee et al. (“A single amino acid in the SH3 domain of Hck determines its high affinity and specificity in binding to HIV-1 Nef protein.” Embo. J. 14(20): 5006-15, 1995) investigated the structural basis of the different SH3 binding affinities and specificities of Hck to the HIV-1 Nef protein and were able to transfer the binding property of Hck SH3 towards Nef to the Fyn SH3 domain by a single mutation in the RT loop of the Fyn SH3 domain (R96I).
Hosse et al. (“A new generation of protein display scaffolds for molecular recognition”, Protein Science, 15:14-27, 2006) specifically address the requirements for binding proteins suitable for therapeutic applications. The authors note the importance of some characteristics for therapeutically useful binding proteins such as serum stability, tissue penetration, blood clearance, target retention and immune response. In the latter respect it is noted that non-human therapeutic proteins should be made as similar to their human counterparts as possible and a human scaffold might be less immunogenic right from the start. These authors conclude:                “However, even an entirely human scaffold is no guarantee for a protein that does not elicit a human immune response, especially if it is an intracellular protein. Randomization of amino acids during library construction can potentially introduce novel T-cell epitopes. Even single point mutations can render a human protein immunogenic. Furthermore, most human scaffolds cause some autoimmune response.”        
Today, the SH3 domains of Abl and Hck kinases are acknowledged as protein scaffolds for generating protein binders with prescribed specificity, even though only binders towards known ligands like the Nef proteins or synthetic peptides have been identified so far (see Binz et al. above).
The SH3 domain of the Fyn kinase (Fyn SH3) comprises 63 residues (aa 83-145 of the sequence reported by Semba et al. (“yes-related protooncogene, syn, belongs to the protein-tyrosine kinase family.” Proc. Natl. Acad. Sci. USA 83(15): 5459-63, 1986) and Kawakami et al. (“Isolation and oncogenic potential of a novel human src-like gene.” Mol Cell Biol. 6(12): 4195-201, 1986). Fyn is a 59 kDa member of the Src family of tyrosine kinases. As a result of alternative splicing the Fyn protein exists in two different isoforms differing in their kinase domains; one form is found in thymocytes, splenocytes and some hematolymphoid cell lines, while a second form accumulates principally in brain (Cooke and Perlmutter, “Expression of a novel form of the Fyn proto-oncogene in hematopoietic cells.” New Biol. 1(1): 66-74, 1989). The biological functions of Fyn are diverse and include signalling via the T cell receptor, regulation of brain function as well as adhesion mediated signalling (Resh, M. D. “Fyn, a Src family tyrosine kinase.” Int. J. Biochem. Cell Biol. 30(11): 1159-62, 1998). It is an intracellular protein. SEQ ID NO: 1 shows the Fyn SH3 sequence (aa 83-145 of Fyn kinase as reported by Kawakami et al. and Semba et al. in 1986, see above):
(SEQ ID NO: 1)GVTLFVALYDYEARTEDDLSFHKGEKFQILNSSEGDWWEARSLTTGETGYIPSNYVAPVDSIQ
The sequence of the RT-Src and the n-Src loop are underlined and double-underlined, respectively.
The amino acid sequence of Fyn SH3 is fully conserved among man, mouse, rat and monkey (gibbon). Chicken Fyn SH3 differs in one, the one of Xenopus laevis in two amino acid positions from the corresponding human domain. Just as other SH3 domains the Fyn SH3 is composed of two antiparallel β-sheets and contains two flexible loops (called RT-Src and n-Src-loops) in order to interact with other proteins.
In summary, the prior art teaches protein frameworks, the so-called scaffolds, as alternatives to established antibody structures. The Src homology 3 domain (SH3) is one of these about 40 or more scaffolds. Among the many different SH3 domains (about 300 in the human genome and several thousands described so far in nature) the Fyn SH3 is one, which has been used once before in order to elucidate SH3 binding specificity and affinity in general. The skilled person is also aware that intracellular proteins are particularly prone to produce immune responses and, therefore, are typically less useful or even useless for in vivo applications like therapy and diagnosis.
The object underlying the present invention is to provide improved target specific and high affinity binding proteins that are suitable as research, and in particular, as diagnostic and medical agents. Furthermore, these binding proteins should be stable and soluble under physiological conditions, elicit little or no immune effects in humans receiving these, and provide a binding structure that is also accessible by large target structures, i.e. that is not masked by steric hindrance.